The present invention relates to a color image pickup device capable of preventing color moire from occurring and relates to color image pickup apparatus using such color image pickup device.
The image pickup devices typically represented by image pickup tube and solid-state image pickup device are widely used in image pickup apparatus. Especially, those color image pickup devices used in single-tube or single-sensor color image pickup apparatus, because of their capability of constructing a color image pickup apparatus by a single image pickup device, greatly facilitates downsizing/power saving of the color image pickup apparatus, since they are characterized in that: a color separation prism is not required and a reduction in size of lens is possible; various adjustments in a multiple-sensor system such as registration are not required; and dissipation power is smaller. In particular, a majority of image pickup apparatus are manufactured as a single-sensor color camera using color CCD image pickup device which is among the solid-state image pickup devices.
In order to obtain color information by a single light-receiving surface, the above described color image pickup devices, without exception, are to perform color modulation (color coding) within the light-receiving surface by using a color filter such as the so-called stripe filter or mosaic filter. For example, a different spectral sensitivity is imparted to each of the pixels by putting filters of the three colors of RGB on the respective photoelectric conversion elements (pixels) in a predetermined, regular order of arrangement. The image signals obtained from an imaging of object thus contain a point-by-point, sequential color information according to such filter array, and the color information can be read out by extracting separated signals by means of separation by each signal corresponding to each filter in accordance with the above described array. Since all pieces of information of RGB are necessary to obtain a luminance signal (Y signal), at least 3 pixels (one each for RGB) are necessary to obtain the luminance information of one pixel. Hence an image can be picked up in colors by using a single image pickup device even though luminance resolution is somewhat sacrificed.
The various types of color coding patterns having been proposed/used as the above described filter array of color filters include: such filter arrays of three primary colors as RGB stripe, Bayer-type RGB mosaic (various types); and such complementary-color filter arrays as YeMgCy stripe, YeMgCyW 4-color mosaic, YeMgCyG 4-color mosaic.
The present invention aims at pointing out and hence providing a remedy for an essential problem which is inherent to the above described color image pickup device irrespective of its electrical construction (whether it comprises an image pickup tube or a solid-state image pickup device, whether it is of the CCD type or other types, etc.) and/or the kind of color coding (whether it uses primary colors or complementary colors, whether it uses three colors or four colors, etc.). For this reason, the following description will be explained by way of an example only, unless otherwise specified.
Of the above described known color coding arrays, an example of RGB Bayer array will now be described by way of FIGS. 1A and 1B. The RGB Bayer array contains a basic array consisting of 2×2 or 4 pixels as shown in FIG. 1A. Such basic arrays are sequentially arranged so as to fill a plane as shown in FIG. 1B. This is characterized in that an allocating ratio of pixels respectively for RGB of 1:2:1 is used so that the luminance resolution be enhanced by increasing the density of G which has a greater contribution to luminance signal. Further, since the same order of arrangement is used along the two directions of vertical and horizontal, it is possible, unlike a stripe type filter, to obtain an isotropic resolution. It should be noted that FIG. 1B shows an example of arbitrarily selected, 8×8=64 pixels.
Since, however, the above RGB Bayer array uses a regular order of arrangement in the manner as described above, there has been a great disadvantage that it involves the so-called color moire, a spurious resolution due to the spatial sampling according to such arrangement. In particular, suppose that a black-and-white object originally having no hue contains a cyclic object where luminance change (black-and-white pattern) occurs at the same cycle as the cycle in the above Bayer array. For example, when a row consisting of RG is noticed and considered as one horizontal line, suppose that the object is white at R filters and black at G filters. Since signals to be outputted are equivalent to the signals obtained from a red object without having luminance change, this results in an outputting of hue which originally does not exist. Such false color signal resulting from a pattern consisting of a stripe-like repetition, i.e., color moire occurs in a low-frequency region due to the so-called frequency folding (aliasing) and hence cannot be removed even by an electrical filtering or the like including a color band suppression to be performed at a later stage.
For this reason, the conventional single-sensor color image pickup apparatus necessitates an optical low-pass filter such as of crystal in its optical system in order to secure image quality. This greatly impedes downsizing and/or cost saving. In addition, a lowered image quality due to yet remaining color moire has not been able to be avoided.